Among the various definitions of the word consciousness is awareness, with wakefulness a general synonym.
If someone hits you over the head, knocking you unconsciousness (though still alive), then you're said to be unconscious. But are you also considered to be unconscious when you're sleeping?
One of the key differences (for me) is that a sleeping person can typically react to an alarm clock, whereas a person who has been rendered unconscious through an injury is effectively dead to the world.
If sleep is NOT a form of unconsciousness, what is it? Is a sleeping person still conscious, or is there another term to describe it?